This invention relates to an improved oil shale retorting process and a separation system for recovering spherically-shaped solids, for example, heat carriers, from irregularly-shaped solids, for example, spent shale. In the oil shale retorting process, heat-carrying spherically-shaped solids are rolled down a continually restored, moving inclined surface to separate these heat carriers from irregularly-shaped spent shale. The heat carriers are then recycled through the retorting process.
It has been proposed to use heat-carrying spherically-shaped solids to retort the solid carbonaceous organic solid matter (commonly called kerogen) in oil shale to produce petroleum products. The sphercially-shaped solids are heated and these hot heat carriers are mixed with crushed oil shale. The heat from the hot heat-carrying solids helps to convert the kerogen in the oil shale to oil and gas and produces a mixture of spherically-shaped solids and irregularly-shaped spent shale solids. The sphercially-shaped solids of a significant size are separated from the irregularly-shaped spent shale solids so that the spherically-shaped solids may be heated and recycled through the retorting process. The spherically-shaped solids must be separated in a dry system wherein dust and other emissions into the atmosphere are controlled. In addition, the mixture of spherically-shaped solids and irregularly-shaped spent shale may be separated at relatively high temperatures, e.g., 204.degree. C. (400.degree. F.) to 538.degree. C. (1000.degree. F.). These temperatures place severe limitations on equipment selection and limit the number of pieces of equipment, especially moving parts, which may be used.
A commercial oil shale plant using recycled spherically-shaped solids may retort as much as 50,000 metric tons (about 55,000 short tons) to 118,000 metric tons (about 130,000 short tons) of raw crushed oil shale per day with varying oil yields per ton. This will require recovery and recycle of one to three times as many tons of the heat-carrying spherically-shaped solids per day. The capacity of the system for separating the spherically-shaped solids will, therefore, be quite large while the plant space allocated to the separating equipment will be limited. The amount of spherically-shaped solids being separated and recycled may also fluctuate widely depending on the richness of the raw oil shale and on other process conditions. It is, therefore, desirable that the separating system adequately separate the spherically-shaped solids with a low loss of spherical heat carriers over a wide range of mass flow rates of spherically-shaped solids and spent shale. This combination of objectives is difficult to achieve. For example, a reduction in loss of spherically-shaped solids with the spent shale tends to be offset by an increase in the amount of spent shale retained with the recycled spherically-shaped solids. By the same token, an increase in mass flow rate of the mixture tends to increase loss of spherically-shaped solids, or retention of spent shale, or both. As hereafter mentioned, this combination of objectives is impractical with separating systems which rely on gravity or size differences, and some other proposals tend to increase attrition of the spherically-shaped solids.
In a process of the type described in U.S. Pat. No. 3,844,929, the heat-carrying solids are special porous pellets having a surface area of at least 10 square meters per gram and a size ranging from approximately 0.04 centimeter (0.055 inch) to approximately 1.27 centimeters (0.5 inch). In this process, a combustible deposition is formed on the pellets. This deposition is burned to reheat the pellets. In this process, it is especially important that the pellets be recovered from the spent shale and that only a small amount of spent shale, if any, be retained with the pellets. The pellets are relatively costly and bear the combustible residue which acts as fuel for heating the pellets. In addition, an excessive amount of retained spent shale will interfere with proper gas and solids flow through the burning zone and will foul the burning zone wherein the deposition on the pellets is burned. Yet, these special pellets are especially difficult to separate from spent shale.
Unlike certain of the other solid heat carriers previously suggested, porous-type pellets undergo size reduction as they are cycled through the retorting system; consequently, the size range of the pellets and the spent shale particles overlap and the two particulate materials frequently have relatively similar specific gravities. This renders separating systems relying on gravity or size differences relatively ineffective. In U.S. Pat. No. 3,803,021, a combination elutriation-size separating system is provided, but this combination system has certain disadvantages. As a result, even though the degree of separation required for the pellet process is greater than that required for some other oil shale retorting processes, the porous pellets are particularly difficult to separate from the spent shale by known means which for the most part rely either on size difference, gravity difference, or elutriation velocity difference.
During a search for an improved separating system, it was learned that the manufacturers of lead shot for shotgun shells once used an inclined surface whereon the round shot rolled down fast enough to jump a slot in the surface while the faulty, out-of-round shot rolled down the surface and fell into the slot. In this system, the surface as inclined at an angle such that the faulty shot moves down the surface at a significant rate, but not fast enough to jump the slot. In a system like this, it seems likely that at a high feed rate per unit of surface area, some of the spherical shot will inherently be deflected into the slot. In the lead shot industry of the past, this may not have been too much of a problem since the out-of-round shot was not a waste material. The rejected shot was probably recycled through the melting and shot forming process. In constrast, in an oil shale process the spent shale is waste material which must be disposed of and pellets carried off with the spent shale are lost. The likelihood of loss of pellets with spent shale might be reduced by reducing the feed rate per unit of surface area, but it would require an enormous sheltered surface area to handle the thousands of tons of heat carriers per day that are cycled in an oil shale retorting process and to provide the desired control over dust and other atmospheric emissions and handle the high temperature involved in oil shale retorting. Nevertheless, it was conjectured that the rolling separation concept might be applied to separating spherical heat-carrying solids from irregularly-shaped, laminar-like spent oil shale solids even though the spent shale would not roll like out-of-round lead shot and did not have the high specific gravity of lead. This led to the improved oil shale retorting process of this invention wherein solid, heat-carrying spherically-shaped solids, especially porous pellets, are separated and recovered from spent shale using a unique separation stage. The separation stage untilizes the face that spherically-shaped solids, e.g., porous pellets, will roll down an inclined surface faster than irregularly-shaped, nonspherically-shaped solids, e.g., spent shale, will slide down an inclined surface, and a continuously restored appropriately inclined surface, that is, an appropriately inclined surface and a particle feed arrangement wherein the two move in a single, continuous direction relative to each other. The surface is inclined at an angle such that the irregularly-shaped solids do not move readily down the surface simply as a result of the force of gravity. The continuously restored inclined surface separating system enables the previously recited conditions to be satisfied, that is, for example, it provides a dry, high temperature system not dependent on gravity or size differences and that is flexible and has few moving parts, large capacity, high feed rate, and good separation with low spherical particle loss and size attrition. The system may be used for separating other spherical solids from nonspherical, irregularly-shaped solids, that is, solids with some flat, rough or laminated sides, especially when standard separating techniques relying on density or gravity differences, or on size, would not be as efficient.